fix(vegas): stop the heaviest plugin doubling across the cycle seam

Smooth Weighted Round-Robin spaces repeats well within a pass, but it
schedules the heaviest item first and usually last as well. The strip
loops, so those two are neighbours: the marquee showed the same plugin
twice running at exactly the one join a within-cycle check cannot see.
Observed on a live rig at 28 slots -- gaps of 6, 7, 7, 7 and then 1.

Rotating the list does not fix it. Rotation preserves the cyclic order
exactly, so it moves where the seam is drawn rather than the adjacency
itself; the trailing entry has to be swapped with one from the middle.

The first version swapped with the first slot that merely fitted, which
undid the spacing this exists to protect -- it moved a repeat from a gap
of 7 into a gap of 2, more clumped than the seam had ever been. It now
picks the candidate furthest from any other appearance, so the repeat
lands in the widest gap.

Left alone when no candidate exists. A plugin holding most of the slots
has to neighbour itself, and scheduling it is better than refusing to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

test/test_vegas_live_weighting.py

@@ -160,6 +160,68 @@ class TestTheSchedule:
         schedule = sm._apply_priority_weights(order)
         assert set(schedule) == set(order), set(order) - set(schedule)
 
+    def test_nothing_doubles_across_the_cycle_seam(self):
+        # The strip loops, so the last slot neighbours the first. Smooth
+        # Weighted Round-Robin schedules the heaviest item first and often
+        # last too, which put the one clump the algorithm exists to avoid at
+        # the one place a within-cycle check cannot see.
+        order = ['baseball', 'weather', 'geochron', 'flights', 'stocks',
+                 'oftheday', 'youtube', 'stocknews', 'leaderboard',
+                 'countdown', 'odds', 'f1', 'football', 'music']
+        plugins = {p: FakePlugin() for p in order}
+        plugins['baseball'] = FakePlugin(live=True, declared=5)
+        plugins['football'] = FakePlugin(live=True, declared=3)
+        schedule = _manager(plugins)._apply_priority_weights(order)
+
+        n = len(schedule)
+        doubles = [schedule[i] for i in range(n)
+                   if schedule[i] == schedule[(i + 1) % n]]
+        assert not doubles, "%r repeats across the seam in %r" % (doubles, schedule)
+
+    def test_the_seam_repair_keeps_every_slot(self):
+        order = ['a', 'b', 'c', 'd', 'e', 'f']
+        plugins = {p: FakePlugin() for p in order}
+        plugins['a'] = FakePlugin(live=True, declared=4)
+        schedule = _manager(plugins)._apply_priority_weights(order)
+        assert _counts(schedule)['a'] == 4, _counts(schedule)
+        assert sorted(schedule) == sorted(
+            ['a'] * 4 + ['b', 'c', 'd', 'e', 'f']), schedule
+
+    def test_the_repair_uses_the_widest_gap(self):
+        # Moving the trailing repeat into the first slot that merely fits
+        # undoes the spacing: on a 28-slot rotation that turned a gap of 7
+        # into a gap of 2, which is more clumped than the seam ever was.
+        order = ['a'] + ['p%d' % i for i in range(13)]
+        plugins = {p: FakePlugin() for p in order}
+        plugins['a'] = FakePlugin(live=True, declared=4)
+        schedule = _manager(plugins)._apply_priority_weights(order)
+        at = [i for i, p in enumerate(schedule) if p == 'a']
+        gaps = [b - a for a, b in zip(at, at[1:])]
+        gaps.append(len(schedule) - at[-1] + at[0])
+        ideal = len(schedule) / len(at)
+        assert min(gaps) >= ideal / 2, "gaps %r for ideal %.1f" % (gaps, ideal)
+
+    def test_an_unavoidable_double_is_left_alone(self):
+        # Five of seven slots are the same plugin, so it must neighbour
+        # itself. Better to schedule it than to refuse or loop forever.
+        order = ['a', 'b', 'c']
+        plugins = {p: FakePlugin() for p in order}
+        plugins['a'] = FakePlugin(live=True, declared=5)
+        schedule = _manager(plugins)._apply_priority_weights(order)
+        assert _counts(schedule) == {'a': 5, 'b': 1, 'c': 1}, _counts(schedule)
+        assert set(schedule) == {'a', 'b', 'c'}
+
+    def test_a_schedule_too_short_to_repair_is_returned_as_is(self):
+        sm = _manager({})
+        assert sm._unclump_seam(['a', 'a']) == ['a', 'a']
+        assert sm._unclump_seam(['a']) == ['a']
+        assert sm._unclump_seam([]) == []
+
+    def test_a_schedule_with_no_seam_clash_is_untouched(self):
+        sm = _manager({})
+        plain = ['a', 'b', 'c', 'a', 'd']
+        assert sm._unclump_seam(plain) == plain
+
     def test_repeats_are_spread_not_clumped(self):
         # The point of Smooth Weighted Round-Robin. Three-in-a-row followed by
         # a long silence would be worse than not boosting at all.